The invention relates to an inert composite electrode, in particular an anode for molten salt electrolysis, e.g. for the extraction of aluminium, magnesium, sodium, lithium, etc., consisting of an active part in the form of a plurality of bar-shaped active elements, particularly of ceramic oxide, which are arranged with their longitudinal axes mutually parallel and in mutually aligned groups, an electrode holder which comprises a current conducting plate with one major surface of which the electrode elements are in firm contact with their end surfaces, and a coupling arrangement which connects the active elements together in groups and holds them in contact with the plate.
In molten salt electrolysis, e.g. for production of aluminium, intensive development is in progress to employ so-called inert anodes, which consist in particular of ceramic oxide, instead of self-consuming anodes of carbon.
A series of advantages provides an incentive for this development:
In manufacture and in operation, the inert anode shows energy savings.
In addition, raw material is saved. In the manufacture, it is unnecessary to have recourse to the fossil material petroleum, from which then petrol, carbon and pitch is produced. In operation of the inert anode, no consumption or only a very slight consumption of anode material occurs. As a result, furthermore the investment and operational costs for the anode plant are avoided.
Since anode replacement which is regularly necessary with consuming anodes can be avoided, the cells can be operated in a closed condition. The working conditions are therefore improved.
The exhaust gas from the cells contains either sulphur dioxide or polyaromatic hydrocarbons. The fluorides can more easily be extracted from the closed exhaust system.
Finally, inert anodes can be operated with higher current densities than carbon electrodes. As a result, the production capacity is increased with a smaller area and/or in less time.
Constructively, the inert electrodes must on the one hand overcome the handicaps of the already existing cells equipped with carbon electrodes. This applies in particular with reference to the current feed and the arrangement and/or the dimensioning of the active components of the anode. But on the other hand, of course also the requirements which result from the material from which the active parts of the inert anodes consist, must be taken into account. This applies in particular with reference to the physical parameters and the manufacturing technology.
An inert composite electrode of the type defined in the introduction is known from DE-PS No. 30 03 922. This consists in essence of an active part, an electrode holder and an arrangement for connecting the two first-named constructional groups together.
The active part is formed from a plurality of bar-shaped active elements. These are arranged with their longitudinal axes parallel to one another and in mutually aligned groups. The overall cross-section perpendicular to the longitudinal axes of the active elements corresponds approximately to the corresponding cross-section of conventional carbon electrode for molten salt electrolysis cells. The individual active elements consist of a ceramic oxide material. For holding the active elements and for current feed to these, a tubular carrier is provided. In this, a further tube is concentrically arranged whose lower end is provided with a bottom plate. This bottom plate has a central hole through which a bar-shaped current feed is introduced whose lower end, finishing beneath the bottom plate, is provided with a current supplying pressure plate. With this pressure plate, the upper end surfaces of the active elements are brought into firm mechanical and electrical contact. For this purpose, the grouped and active elements each have in their upper section a respective hole which is likewise aligned to that of another group. Through these mutually aligned holes a suspension rod is put in each case, the ends of which contact a support plate. This support plate and the said bottom plate are braced by screw bolts whereby the upper end surfaces of the active elements are brought into contact with the current feeding pressure plate. If necessary, between the end surfaces of the active elements and the pressure plate, an intermediate layer having good electrical conductivity can be inserted.
This known electrode construction has several severe disadvantages.
First of all, its construction is as a whole relatively complicated, in particular with reference to the suspension rods which are put through the holes in the head section of the active elements and must be mounted and braced accordingly.
Furthermore, the manufacture of the holes in the head sections of the active elements requires considerable manufacturing expediture. They can only be produced in the green condition of the ceramic oxide active elements. Furthermore, holes are associated with greater tolerances, in particular having regard to the alignment of the active elements arranged in groups, since such tolerances occur already in the manufacture of the active elements in the green condition and furthermore further dimensional changes unavoidably occur during sintering of the active elements. This has the consequence that the holes of one group of active elements are not exactly aligned so that some of the active elements placed in a row on a suspension rod fail to make contact or make only insufficient contact at their end surfaces with the current feeding plate of the electrode holder. This applies then all the more so in operation where the various expansion coefficients of the material of the active elements on the one hand and the current feeding plate on the other hand have a pronounced negative effect on the contact between the end surfaces of the active elements and the plate. This results in increased voltage drop with the consequence that the electrical efficiency drops.
This disadvantage is exacerbated in that the holes reduce the cross-sectional area parallel to the longitudinal axes of the active elements, that is to say in the cold region of the active elements. As a result, the current paths are restricted there.
The mentioned weakening of the cross-section of the active elements of the known anode also reduces the mechanical strength of the active elements and this in a region in which on the one hand the respective suspension rod exerts an increased force on the material of the active elements as a result of its prestressing and on the other hand also the highest tension forces appear as a result of the weight of the active elements. As a result of this, the largest mechanical stresses occur just in the region of the weakest cross-section of the active elements so that an increased danger of fracture of the electrode elements occurs at the said position.
Finally, in the known anode construction there is no or little attention directed to the necessary electrolyte movement in the region of the lower section of the electrode elements inserted into the melt and to the gas discharge in the region of the electrode elements.